We do not need to understand every single word when we read a text. Some words are not crucial for our comprehension and can therefore be ignored. Some unfamiliar words can be inferred from the text context. If they cannot be inferred, they can be looked up in a dictionary. However, ignoring unknown words, guessing them and looking them up in a dictionary cannot compensate for lack of good vocabulary knowledge. It is possible to ignore some unknown words in a novel, or a story, but not in texts dense in academic or professional information, such as medical or legal texts. Even if we can use contextual clues successfully, not all contexts provide clues for unknown words. Moreover, readers often ignore clues when they mistakenly consider unfamiliar words as familiar. Most importantly, clues often appear in words which themselves are unknown to learners and are therefore unusable. (For a discussion of inferring words from context, see Laufer, 1997, 2005.) A good dictionary is certainly helpful for finding the meaning of unknown words, assuming the reader knows how to use the dictionary efficiently. But looking up a large number of words may consume too much time and interfere with reading fluency. Hence, neither guessing nor dictionary use strategies can compensate for insufficient knowledge of the text’s vocabulary.

We can ask two related questions about the minimal lexis necessary for comprehension: (1) What percentage of a text’s vocabulary should readers know to comprehend the text without resorting to compensatory strategies? (2) How large should the reader’s vocabulary be in order to understand the necessary percentage of the text’s words? The minimal vocabulary in terms of the percentage of familiar words in a text and the reader’s vocabulary size is the lexical threshold required for understanding an authentic text.

Why is it important to quantify the lexical threshold? Most researchers agree that only when readers possess a critical mass of second language (L2) knowledge (lexical and grammatical) can general reading skills, such as distinguishing between main and peripheral information, between explicit and implicit material, operate most efficiently (Bernhardt & Kamil, 1995; Carrell, 1991; Clarke, 1980; Lee & Lemonnier-Schallert, 1997). Furthermore, vocabulary knowledge is a good predictor of reading proficiency, if not the best (Bernhardt & Kamil, 1995; Laufer, 1992; Nation 2001, 2006; Qian, 2002; Ulijn & Strother, 1990).

How Much of a Text’s Vocabulary Should Readers Know to Reach Adequate Comprehension?

Laufer (1989) found that the knowledge of 95% of the text’s vocabulary was usually required to score 55% on a comprehension test and suggested that 95% of lexical coverage was the lexical threshold.

This article first appeared in TESOL Quarterly, 47, 867–872. For permission to use text from this article, please go to Wiley-Blackwell and click on "Request Permissions" under "Article Tools."
doi: 10.1002/tesq.140